The present invention relates to improvements in the field of monochromic and polychromic dynamic printing. More particularly, the invention is concerned with an improved method and apparatus for reproducing an image by electro-coagulation of an electrolytically coagulable colloid and transferring the image thus reproduced onto an end-use support, such as paper.
Applicant has already described in his U.S. Pat. No. 3,892,645 of July 1, 1975 an electrolytic printing method and system in which a thin layer of a liquid composition containing a colloid such as gelatin or albumin, water and an electrolyte is interposed between at least one pair of opposite negative and positive electrodes spaced from one another to define a gap which is filled by the liquid composition. In one embodiment, there is a plurality of electrically-insulated juxtaposed negative electrodes and selected ones thereof are electrically energized to pass electric pulses through the layer at selected points to cause point-by-point selective coagulation and adherence of the colloid in variable thickness on the positive electrode directly opposite each energized negative electrode, thereby forming imprints.
It is very important that the gap between the negative and positive electrodes by uniform throughout the active surfaces of the electrodes since otherwise there will be a variation in the thickness of the layer and thus a corresponding variation of the electrical resistance thereof at different locations between the electrodes, which will result in a non-uniform image reproduction as the thickness of the coagulated colloid is proportional to the amount of current passed through the layer. As this gap is of the order of 50.mu., its uniformity is of course very difficult to control. This is especially true in the case where the positive electrode is in the form of a revolving cylinder so as to be also used as a printing roller for high speed transfer of the image reproduced onto paper or the like; such a cylinder must be of high precision and have a cylindrical surface virtually free of any defects, and is thus very costly. Moreover, since the negative electrodes are generally energized more than once in the reproduction of an image, these become polarized resulting in a gas generation and accumulation at the negative electrodes, which adversely affect the image reproduction.